Structural evolution of an early Proterozoic strata-bound Cu-Co-Zn deposit, Outokumpu, Finland

Abstract

The massive pyrite-pyrrhotite-chalcopyrite-sphalerite deposit of Outokumpu, comprising the Keretti and Vuonos orebodies, is a deformed and metamorphosed strata-bound mass associated with mineralised stockworks. Mobilisation of much of the ore followed formation of large recumbent isoclinal folds that are the major structures of the surrounding rocks and associated with the modification of originally flat saucer-shaped ore lenses into elongate ruler-shaped masses. Further modification of shape took place at the mobilisation stage with much of the pyrrhotitic ore, particularly, now occupying the thickest parts of the orebodies in the form of breccia or microbreccia. In many parts gross original characters still exist and the pyritic and pyrrhotitic constituents of the ore have survived as separate entities while locally the pyritic ore retains pre-deformational characteristics and consistent stratigraphic position within a thin horizon.Both ore and country rocks show evidence of extensive polyphase deformation with the effects of six fold phases shown in the ore. Mineral assemblages in the country rocks indicate a middle amphibolite facies peak of metamorphism. The serpentinite-black schist-carbonate-quartzite rock assemblage, with which the ore is associated, was tectonically incorporated within the regionally extensive mica schist by even earlier subhorizontal thrusting. This is related to the movement of a thrust nappe with the interdigitation of an ocean-floor ophiolite assemblage and flysch deposited during ocean closure associated with Svecokarelian tectonism.The original formation of the Keretti and Vuonos sulphide masses took place in a marine exhalative environment with a pyritic layer overlying a pyrrhotitic layer in each of the two c. 4 km diameter irregularly oval-shaped depressions whose centres were c. 8 km apart. The mineralised stockwork below each mass represents the upper parts of the conduit for metalbearing fluids in a convective system.

Metamorphic development of cordierite-bearing layered schist and mica schist in the vicinity of Savonranta, eastern Finland

“…1) form part of the Kalevian schists in the nomenclature of Nykänen (1975) and, on the basis of regional structural relationships, can be subdivided into two nappe units, the Outokumpu and the Savonranta nappes, the latter being uppermost (Koistinen 1981;Halden 1982 b;Park and Bowes 1983, fig. 18).…”

“…2). Also shown is the distribution of aluminosilicate polymorphs, garnet and cordierite (the latter indicates the position of the layered schist); data from Nykä-nen (1975) and Koistinen (1981) included; comp. foln.…”

“…1;Bowes 1976;Koistinen 1981), and used by Halden (1982 a) in describing the inter-relationship between folds (FL .... F laxe ), and their associated axial planar fabrics (S, .... S LAXE ) developed in the mica schists in the vicinity of Savonranta. Pre-D[ refers to a deformational phase that preceded the D, deformational phase; this includes major thrust movements that resulted in the tectonic interdigitation of rocks from different crustal levels and different crustal segments (c.f; Koistinen 1981;Park and Bowes 1983).…”

“…Campbell et al 1979;Koistinen 1981;Treloar et al 1981;Halden 1982 a;Park 1983 a, b) means that the concept of a general increase in metamorphic grade from E to W in central Finland (Huhma 1975) needs amplification and integration with studies of mineral reactions and successive mineral growths throughout the region. This can be achieved by linking changes in P-T conditions with changes in structural features from district to district and so establishing phase by phase regional variations through the polymetamorphic sequence on the basis of correlation of coevally developed minerals.…”

Sm-Nd data for mafic-ultramafic intrusions in the Svecofennian (1.88 Ga) Kotalahti Nickel Belt, Finland - Implications for crustal contamination at the Archaean/Proterozoic boundary

“…It has been suggested that the composition of the parental magma was basaltic with a MgO content of about 12 wt.% (Peltonen, 1995a(Peltonen, , 2005. Emplacement of magma was accompanied the gently dipping D 2 folding and thrusting event (Mäkinen & Makkonen, 2004;Makkonen, 2005), which was a response to convergence and thrusting of Svecofennian terranes over the Archaean cratonic foreland (Koistinen, 1981). During D 3 the subhorizontal D 1-2 structures were reoriented into steeper orientations (Mäkinen & Makkonen, 2004).…”